The present invention is in the field of methods of purifying organic compounds, including but not limited to those organic compounds which are the final products and intermediates, especially the latter, produced by synthetic methods in organic chemistry. In particular, the methods are for purifying alkyl esters of organic compounds which are carboxylic acids. The present invention relates to an improved method of purifying by phase separation (6-chloro-2-carbazolyl)methyl-malonic acid di(C.sub.1 -C.sub.4 alkyl) esters, especially the diethyl ester, which is sometimes referred to hereafter as the "carbazole ester", although this term is also used as a general reference to all of the di(C.sub.1 -C.sub.4 alkyl) esters involved in the method of the present invention.
The carbazole ester is the starting material for one process of making carprofen, a highly effective COX-2 selective anti-inflammatory drug approved by the Food and Drug Administration, Committee on Veterinary Medicine (FDA/CVM) for use in dogs in the United States. The carbazole ester starting material is known to potentially contain at least one impurity, created during one step of a related manufacturing process, which may comprise as much as 0.9% by weight of the carbazole ester starting material. The composition of this impurity is discussed in detail further below, but the purification method of the present invention is contemplated to include within its scope not only this impurity but other impurities as well. In order to obtain carprofen final product in sufficiently pure form for use as an animal drug, all such impurities must be reduced to a minimum.